The Castillo is a masonry star fort made of a stone called "coquina", literally "little shells", made of ancient shells that have bonded together to form a type of stone similar to limestone.
In November 1702, British forces attacked the fort. All of the city's residents, some 1,200 people, along with all of the fort's soldiers, some 300, were protected inside the wall of the fort for the next two months during the attack. The British cannon had little effect on the walls of the fort. The coquina was very effective at absorbing the impact of the shells, allowing very little damage to the walls.